Coal-rock sensing device



Jan. 2, 1962 s. l. PERSSON ET AL 3,015,477

COAL-ROCK SENSING DEVICE Filed Aug. 20, 1958 4 Sheets-Sheet 1 RESISTANCElN OHMS COAL I04 I ROCIVK" (SANITSTONE) o 5 IO I5 20 V 0 U S INVENTORSSTEN 1. PERSSON FIG. 2 BY CHARLES H. REYNOLDS Jan. 2, 1962 s. l. PERSSONET AL 3,015,477

COAL-ROCK SENSING DEVICE Filed Aug. 20 1958 4 Sheets-Sheet 2 R R R C R s8 7 6 5 4 3 2 I O .r m moz km wmm J Ou xOOI H w VOLTS FIG.

INVENTORS PERSSON By CHARLES H. REYNOLDS STEN FIG. 4

Jan. 2, 1962 s'. l. PERSSON ET AL 3,015,477

COAL-ROCK SENSING DEVICE Filed Aug. 20, 1958 4 Sheets-Sheet 5 +4OV 250 Vi- Y 54 i g 58 57 eo ROCK COAL l 2 FIG. 6

INVENTORS STEN I. PERSSON BY CHARLES H. REYNOLDS AT ORNEY Jan. 2, 1962Filed Aug. 20 1958 S. l. PERSSON ETAL COAL-ROCK SENSING DEVICE 4Sheets-Sheet 4 "I 70 AVERAGE 0c LEVEL e '7l' AVERAGE DC LEVEL Fl G 7INVENTORS STEN l. PERSSON BY CHARLES H. REYNOLDS United States PatentOllice 3,015,477 Patented Jan. 2, 1962 3,015,477 COAL-ROCK SENSENGEEVICE Sten l. Persson and Charles H. Reynolds, Rochester, N.Y.,assignors to General DynamicsCorporation, Rochester, N.Y., a corporationof Delaware Filed Aug. 2t), 1958, er. No. 756,183 Qlaims. (Cl. 262-47)This invention relates to the art of coal mining and is particularlydirected to means and methods of detecting rock at the boundaries of acoal'seam.

Coal is usually found in generally flat horizontal layers or seams ofvarious vertical depths. The seam is entered from one edge with cuttingtools or drills to break, loosen, and remove the coal. The floor androof of the seam is usually of minerals much harder to out than coal andseri- "ously shortens the life of the cutting tools when the roof orfloor rock is accidentally. entered by the tools.

Attempts have meen made in the past to measure the electrical resistanceof the strata in which the cutting tools are working. Electricalresistance, however, has proven heretofore to be a poor criteria foridentifying bore-hole materials because of the many uncontrolledvariables which affect resistance. The presence and amount of moisturenear the test probes changes resistance widely, for example.Quantitative resistance values cannot, as a practical matter, beobtained because reference values of resistance cannot be assigned. Theproblem of identifying minerals by resistance measurements is generallynot feasible because in practice the resistance varies from near zero toinfinite values.

An object of this invention is to provide improved methed and means forfinding the boundary between two dissimilar mineral formations in theirnative state.

A more specific object of this invention is to provide improved methodsand means for sensing rock formations at the boundaries of coal seams,particularly in connection with the operation of continuous miningmachines.

The objects of this invention are attained by mounting one or two testprobes adjacent the cutting bits of a coal mining machine in such afashion that the probes follow the bits and alternately contacts thecoal and the rock during cutting operation. The probes are electricallyconnected in a bistable circuit of such a character that the resistanceof the minerals effectively connected between the probes causes thebistable circuit to assume either of two characteristic stable states.The potential applied between the probes is chosen at an optimum,relatively high, value at which the ratio of resistances of the twominerals is at a maximum, so that as the probes pass alternately throughthe two minerals two distinct signals are generated by the bistablecircuit.

Other objects and features of this invention will become apparent tothose skilled in the art by referring to the specific embodiments ofthis invention described in the following specification and shown in theaccompanying drawing in which:

FIG. 1 is a fragmentary view of a mining machine including a sensingprobe or cutter of this invention;

FIG. 2 is a voltage-resistance graph of typical minerals;

FIG. 3 is a graph showing values of the ratio of resistances of coal androck, plotted against voltage;

FIG. 4 is a circuit diagram of a rock sensing device embodying tube-typeamplifiers;

FIG. 5 is a circuit diagram of a rock sensing device embodyingtransistor amplifiers;

FIG. 6 is a circuit diagram of a sensing device embodying transistors ina multivibrator sensing type circuit; a d.

FIG. 7 is a pulse chart of the output of the multivibrator of FIG. 6.

In FIG. 1 is shown a portion of a coal boring machine having a wheel orarm 10 rotating on a center, not shown, for carrying cutting toolshorizontally forward in a coal vein. The vein contemplated here is ofthe type generally found throughout the central United States at variouslevels below ground. The vertical depth of the vein will typically varybetween three to six feet. The floor and ceiling of the vein comprisesan underlay and overlay of rock generally much harder than the coal. Therock may be classified-as slate; shale, boney, fire clay, or sandstone.lron pyrites sometimes appear with the coal vein. Rock is generallyStratified as indicated at 11 in FIG. 1, while the coal is shown at 12.Obviously the cutting tool siwinging on a center within the vein willdig into the rock in the floor or ceiling if the mining machine driftsup or down from its approximate center position. The objects of thisinvention are attained by sensing the electrical resistance of the coaland comparing that resistance with the resistance of the rock so thatthe cutting tool or sensing probe adjacent the cutting tool passesalternately from coal to rock to coal.

On the end of arm 10 is mounted the sensing tool or probe 13 with ahardened lead edge and with a shank l4 clamped in a mechanically strongbushing 15 of insulating material. The sensing tool and its insulatingsupport must be rigidly mechanically attached to the mining machine, aswith the pillow block 16. It is important that the sensing tools 13 makeintimate contact with the strata being cut and although a cuttin typebit is shown a spring-pressed probe with a rounded nose could, ifdesired, be brought in contact with the strata immediately behind thestandard cutting tools of the machine.

Further, although a single sensing tool is shown in FIG. 1, it iscontemplated that two tools a distance apart may be used. If a singletool is used, the electrical circuit to the tool is completed throughthe frame of the machine. Where the sensing tool is mounted on arotating head, slip rings must be usedto complete electricalconnections.

Contrary to expectations, the electrical resistance in ohms of mostbituminous coal is many times higher than the resistance of most rockformations. too, it was found that the electrical resistance of bothrock and coal is a function of the voltage employed to make theresistance measurement. No ready explanation can be offered for thisunusual phenomenon. In FIG. 2, for example, the resistances of asandstone type rock and of a common bituminous coal are plotted forvarious voltages. That is, the resistance of rock and coal is plottedfor various values of direct current voltage from zero to 20 volts. Itcan be seen that in the specific example considered, the resistance ofrock and coal is the same at about 2 volts. As the voltage increased upto about 7 volts the resistance of the rock dropped sharply,

and at voltages above 7 volts the resistance remains substantiallyconstant. This finding suggests that the resistance measurements must bemade at relatively high voltages and in the region where theresistances. are comparatively stable. In the case of the standstone andcoal samples of FIG. 2, the ratio of coal and rock resistance is largestin the 7 to 20 volt region. Fortunately, the

high ratio of coal resistance to rock resistance is effectivelySurprisingly,

concerned because of the high impedance of the circuitry. Specificallydifferent coal and rock minerals would, of course, yield specificallydifferent resistances and resistance ratios but the order of resistancesand their ratios would be about the same and there would be one optimumtest voltage.

The lead 17, FIG. 1, from the sensing tool 13 is connected, as shown inFIG. 4, to the input of a bistable circuit which, according to thisinvention, produces two distinct signals as the sensing tool 13 movesfrom contact with coal to contact with rock and back again. Lead 18 isconnected either to the frame of the machine or to a second sensing tooleither of which is considered the reference ground for the system. Theparticular bistable device shown in FIG. 4 comprises a boot strap-typecircuit having triode amplifiers 20 and 21. Both amplifiers areconnected across the bus bars 22 and 23, the latter being grounded whenthe lead 18 is connected thereto. Windings of the relay 24 are connectedin series with the anode and the cathode resistor 25 is connected inseries with the cathode. The grid of amplifier 20 is connected to anintermediate point on the potentiometer comprising resistances 26 and 27which are so proportioned that tube 20 is cut off in one stable state,the cutoff gridcathode voltage being made amply high by the normallyhigh current flowing in amplifier 21 and its series-com nectedresistance 25. That is, normal high current through 21 and 25 will holdthe cathode of 20 at a relatively high positive potential, thus assuringcutoff. If now the sensing tool resistance in shunt with 27 is suddenlyincreased, as when the sensing probe enters coal, the grid of 20momentarily rises unblocking amplifier 20, permitting the cathode end ofresistance 25 to rise and the anode end of winding 24 to drop. These twoeffects applied respectively to the cathode and grid of amplifier 21reduces the current flow through 21 which in turn reduces the biasvoltage generated across 25, the regenerative action producing a runawayeffect that positively and abruptly reverses the normallynonconductiverconductive condition of amplifiers 20 and 21. Withamplifier 20 now conducting, winding 24 is energized and armature 28 ismoved to illuminate lamp 22 turning off lamp 341. The two lamps may becolored distinctively, say, red and green so that the operator candetermine momentarily whether his cutting bit is in rock or coal.

When the resistance across 27 momentarily decreases, as when the sensingtools enter rock, the grid bias of 20 momentarily changes to a cutoffvalue permitting amplifier 21 to momentarily conduct and to trigger theaction which results in regenerative amplification and sudden andpositive restoration of the normal nonconducting condition of amplifier20 and the conducting condition of amplifier 21.

Resistance 27 is made variable so as to easily control the thresholdresistance in the grid circuit which will cause the shift fromconductive to nonconductive state, or vice versa.

The circuit of FIG. is the equivalent of the circuit of FIG. 4 insofaras the philosophy of operation is concerned but with transistorssubstituted for the vacuum tube type amplifiers. The transistors foundparticularly useful for this bistable device is of the P-N-P typeconnected between ground and a positive 20 volt bus connected to theemitters. The collector of transistor 40 is coupled to the base oftransistor 41, while the emitter of 40 is coupled to the emitter of 41.Transistor 40 is normally cut off by virtue of the normally highresistance between the sensing tools, labeled 13 and 13a in FIG. 5. Thesensing tools are connected in the potentiometer circuit to which thebase of 40 is tapped, and one leg of which comprises adjustableresistance 42. Transistor 41 is normally conducting and supplies thebias across resistance 45 for holding transistor 40 nonconducting. Whenthe resistance at 13-13a suddenly drops as when rock is entered, thebase of 40 drops, the collector of 40 and base of 41 rises, and thecurrent through 41 momentarily drops. Regenerative amplification is thesame as in FIG. 4 and full stable conducting current abruptly flowsactuating relay 44. Operation of armature 48 switches the lamps 49 and50 and the operator instantly is warned of the entry of his cutting bitinto the new strata. The two lamps will alternately illuminate as thecutting bit enters rock and coal, and the operator can promptly respondby appropriately steering the mining machine. Resistance 42 is adjustedso that the base bias will shift across the required threshold value forthe particular rock-coal strata being cut.

Alternate cutting in rock and coal can likewise be displayed to theoperator by the multivibrator type circuit of FIG. 6. The characteristicof the circuit of FIG. 6 is that two stable states of the amplifiers arenormally switched back and forth at one predetermined frequency and thatthis frequency abruptly changes to a new and distinct frequency as thesensing tool passes into a different mineral formation. That is,frequencies and f of the bistable multivibrator appear at the output ofthe circuit immediately upon the cutting bit entering rock and coal,respectively. The output circuit comprises the relay winding 54,operating armature 58 and lamps 59 and 60. In this case, energization ofrelay 54 is obtained by integrating the higher frequency pulses of themultivibrator. Only frequencies above a certain value will containsufficient energy and current to operate the armature. At this certainfrequency, the relay is preferably designed to operate positively andunambiguously. This particular mode of operation can best be evaluatedby referring to FIG. 7. If the frequency 1; produces an average directcurrent in the winding represented by level 70, obtained by integratingareas under the current loops, the spring or gravity bias of thearmature 58 can be made sufficient to prevent operation. When thefrequency of the multivibrator increases to or above f however, theaverage direct current level rises to 71 which can easily be adjusted tooperate the relay armature and cause the desired response in the lamps.

The specific multivibrator shown comprises transistors 72 and 73, thecollector of one being capacitively coupled, as shown, to the base ofthe other. The RC time constant circuits of each transistor isdetermined respectively by resistance 74 and condenser 75 on the onehand, and resistance 76 and condenser 77 on the other hand. Theresistance between the sensing tools are, in FIG. 6, connected inparallel with resistance 76 so that minerals of different resistanceacross the probes unbalances one RC time constant with respect to theother. The pulse repetition rate when the sensing tools are in rock maybe, for example, 100, while the pulse repetition rate for sandstonecould be made 500, or any frequency distinctly different than the other.Since one time constant remains fixed and the other variable, pulsewidths remain fixed while the frequency changes. However, either or bothtime constants are preferably variable, as by variable resistances 74and 76 to increase the sensitivity of the device and accommodate thedevice to specifically different rock-coal formations.

The rock-coal sensing device of this invention comprises a bistableamplifier device which instantly indicates to the operator the presenceof his cutting bit in rock or coal. Advantage is taken of the fact thatat a certain relatively high test voltage, a large ratio of resistancesof dissimilar minerals can be observed and employed to reliably triggera bistable multivibrator. Many modifications may be made in the specificcircuit details without departing from this central thought of theinvention.

What is claimed is:

1. In combination in coal mining machinery, at powerdriven cutting bitfor cutting coal in a natural coal seam having a rock boundary; andmeans for sensing rock encountered by said bit, said means comprising asensing tool disposed adjacent said bit for intimately and alternatelycontacting the face of the coal and rock being cut, a resistancesensitive bistable device having a trigger circuit, said trigger circuitbeing connected directly to said sensing tool and being responsive tothe electrical resistance of the coal and rock being cut for abruptlychanging the bistable device from one state to another as saidelectrical resistance changes from one value to another, respectively,above and below a predetermined resistance value, and means for manuallychanging the level of said predetermined resistance value.

2. A system for continuously indicating the strata being cut by thecutting tool of a coal bore mining machine having a rotary cutting head,said system comprising a strata cutting tooth mounted on said head,means for alternately and instantaneously measuring and indicating theelectrical resistance of the two principal strata being cut, said meanscomprising a test probe mechanically mounted on and electricallyinsulated from said rotary cutting head and including a source ofvoltage connected to said probe and of a value for producing a maximumresistance ratio between the two principal strata being cut, a bistablecircuit connected to said probe and responsive to the two resistances ofsaid ratio for producing, respectively, the two stable states of saidbistable circuit, and means for distinctively indicating each stablestate.

3. In combination, a bore mining machine having a rotary cutting headadapted to be advanced into desired strata, means for indicating achange in the strata being cut comprising a cutting tooth mounted onsaid rotary head in cutting relation to the strata being cut and meansfor sensing successive electrical resistances in the difierent strata asthe cutting tooth moves alternately through the strata; the sensingmeans comprising a bistable amplifier device for producing current oftwo distinct levels, said amplifier device being connected to saidcutting tooth and being responsive to said electrical resistances in thedifferent strata to shift from one stable state to the other, and meansfor adjusting said amplifier device to change the threshold value ofelectrical resistance across which the strata resistance must change toshift said stable states.

4. in combination, a bore-type coal mining machine with a rotary cuttingtool, a rock-coal sensing device for operation with said coal miningmachine, said device comprising sensing means for continuously andinstantaneously sampling the electrical resistance of mineral stratabeing out immediately adjacent the cutting region; a bistable amplifiermeans for generating two distinct signals, and for shifting from onestable state to the other, in response to two electrical resistancevalues, a resistance sensitive shift controlling circuit in saidamplifier means, means for connecting the sensing means into said shiftcontrolling circuit for causing a shift each time said sensing meanspasses through the boundary between two different mineral strata ofdifferent electrical resistance.

5. In combination, a bore mining machine having a cutting bit for tunnelboring along a seam of natural coal lying between a floor and ceiling ofrock, and including test probes for engaging the open face of the tunnelbeing bored, and a rock-coal sensing system comprising a flipflop typebistable amplifier, a potentiometer with a plurality of resistancesconnected across a voltage source, the trigger circuit of said bistableamplifier being connected to an intermediate point on saidpotentiometer, one resistance of said potentiometer being manuallyadjustable for controlling the threshold triggering voltage of saidbistable amplifier, and means for connecting said test probes inparallel with saidone resistance for dynamically shifting the potentialof said intermediate point on the potentiometer to either side of saidthreshold triggering voltage.

References Cited in the file of this patent UNITED STATES PATENTS

